Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 1 Jun 2018 11:09:42 +0200
From:   Petr Mladek <pmladek@suse.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     Maninder Singh <maninder1.s@samsung.com>,
        sergey.senozhatsky@gmail.com, rostedt@goodmis.org,
        linux-kernel@vger.kernel.org, a.sahrawat@samsung.com,
        pankaj.m@samsung.com, v.narang@samsung.com
Subject: Re: [PATCH 2/2] printk: make sure to print log on console.
Message-ID: <20180601090942.ek3j4bpbhschljrw@pathway.suse.cz>
References: <CGME20180531102246epcas5p2f1cbc6ff217172e12e2f78bb88eb4a7e@epcas5p2.samsung.com>
 <20180531102246epcas5p2f1cbc6ff217172e12e2f78bb88eb4a7e~zs5h59tMh2250222502epcas5p2S@epcas5p2.samsung.com>
 <20180531105215.GF477@jagdpanzerIV>
 <20180531122112.bfeqtmwpl2qc67a5@pathway.suse.cz>
 <20180601044050.GA5687@jagdpanzerIV>
 <20180601085356.kncuat7epkbtythv@pathway.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180601085356.kncuat7epkbtythv@pathway.suse.cz>
User-Agent: NeoMutt/20170421 (1.8.2)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri 2018-06-01 10:53:56, Petr Mladek wrote:
> On Fri 2018-06-01 13:40:50, Sergey Senozhatsky wrote:
> > On (05/31/18 14:21), Petr Mladek wrote:
> > > > 
> > > > Upstream printk has no printing kthread. And we also run
> > > > printk()->console_unlock() with disabled preemption.
> > > 
> > > Yes, the comment was wrong
> > 
> > Yes, that was the only thing I meant.
> > I really didn't have any time to look at the patch yesterday, just
> > commented on the most obvious thing.
> 
> Fair enough.
> 
> > > but the problem is real.
> > 
> > Yep, could be. But not exactly the way it is described in the commit
> > messages and the patch does not fully address the problem.
> > 
> > The patch assumes that all those events happen sequentially. While
> > in reality they can happen in parallel on different CPUs.
> > 
> > Example:
> > 
> > 	CPU0					CPU1
> > 
> > 	set console verbose
> > 
> > 	dump_backtrace()
> > 	{
> > 		// for (;;)  print frames
> > 		printk("%pS\n", frame0);
> > 		printk("%pS\n", frame1);
> > 		printk("%pS\n", frame2);
> > 		printk("%pS\n", frame3);
> > 		...				console_loglevel = CONSOLE_LOGLEVEL_SILENT;
> > 		printk("%pS\n", frame12);
> > 		printk("%pS\n", frame13);
> > 	}
> > 
> > Part of backtrace or the entire backtrace will be missed, because
> > we read the global console_loglevel. The problem is still there.
> 
> [...]
> 
> > So I'd say that most likely the following scenarios can suffer:
> > 
> > - NMI comes in, sets loglevel to X, printk-s some data, restores the
> >   loglevel back to Y
> > - IRQ comes in [like sysrq, etc] comes in and does the same thing
> > - software exception comes in and does the same thing [e.g. bust_spinlocks()
> >   at arch/s390/mm/fault.c]

I forgot to say that it was a great point and analyze.

> My view is:
> 
> The race with another printk() (console_lock owner) is much more
> likely than a race between two CPUs manipulating console_loglevel.
> 
> The proposed patch seems to be in the right direction. It is supposed
> to fix the most likely scenario. We could block it and request full
> solution but I wonder if it is worth it.
> 
> I am personally fine with this partial solution for now. We could
> always make it better if people meet the other scenarios.

I am still fine with the partial solution. Well, I will think
more about it before approving any patch.

Best Regards,
Petr